You mention allocating stock to a Purchase Order, so I have to assume you are talking about your Customer's PO and you have entered it as a Sales Order in Business Central. Is this correct?
If so, if you have reserved your available inventory to a demand source, such as a Sales Line, the Quantity on Hand does not change but the Item Availability does. Business Central is tracking how much inventory is available AFTER reservations, but it doesn't change how much is sitting on your shelf (Quantity). The reason for this is your Inventory Value is based on product on the shelf, regardless of whether there is a reservation to an Order.
You can use the "Item Availability by Event" to see the impact of sales and purchases against your Quantity on Hand:
This will show you, by Item, what are the future changes in inventory based on what is entered in your system.
If you are doing replenishment planning (especially for all Items), I would suggest using the Planning Worksheet. This requires setting your planning criteria and reordering policies on the Items you wish to plan for. This is a fantastic tool to improve the management of your inventory.
Hopefully this helps get you some more information out of your system.
Best Regards,
Ben Baxter
Accent Software Inc